A marine jet propulsion system includes an inlet duct, a pumping means and a nozzle. The inlet duct delivers water from under the hull to the pumping means, which is driven by an engine. The pumping means delivers the water through the nozzle, which produces a water jet, thereby propelling the watercraft through the body of water in which the watercraft moves. In the prior art, a reversing bucket redirects the jet flow back under the boat fully for reverse thrust and partially for neutral thrust.
My U.S. Pat. Nos. 5,658,306, 5,679,035 and 5,683,276, which are incorporated by reference, disclose systems and methods for simultaneously optimizing the hydraulic efficiency of the inlet duct and the pumping means. Such increased hydraulic efficiency has allowed a substantial increase in the design system flow rate, which is well understood in the propulsion field of art to improve propulsion efficiency at low watercraft speeds. The increased hydraulic efficiency of the system and the methods preserves propulsion efficiency at higher watercraft speeds, so that the systems operate more efficiently over a wide range of boat speeds and accelerations.
From disclosures in my US Patents and through common knowledge in the propulsion field of art, it is known that larger mass flow rates and concomitantly lower nozzle velocities are more efficient at lower watercraft speeds, whereas lower mass flow rates and concomitantly higher nozzle velocities are more efficient at higher watercraft speeds. To achieve these ends, it is well understood in the art that a larger nozzle area is useful at low watercraft speeds, whereas a smaller nozzle area is most useful at higher watercraft speeds. Such reduction of nozzle size with watercraft speed was a natural consequence of the operation of the systems and the methods disclosed in my US Patents. However, a greater reduction of nozzle size with watercraft speed would be desirable for increased propulsion efficiency over a range of watercraft speeds.
When the watercraft is operating in a planing mode, the water jet obliquely strikes the water surface behind the watercraft, which results in turbulence on the water surface. Such turbulence is dependent on the velocity of the water jet relative to the water surface. When the velocity of the water jet relative to the water surface is high, as is common in the prior art, the water jet interacts with the water surface to produce a high turbulent spray of water behind the boat, which is commonly called a “rooster tail.” The rooster tail is commonly considered objectionable for water skiing and wakeboarding behind the watercraft. Reducing the velocity of the water jet relative to the water surface eliminates the rooster tail, but still leaves a turbulent trail of surface water in the wake of the watercraft, which is still objectionable to wake boarders, who like to use short ropes. A further reduction of the velocity of the water jet relative to the water surface would be desirable for the further reduction of the turbulent trail of surface water in the wake of the watercraft.
Another shortcoming of the prior art is the fact that the engine commonly operates at substantially higher rpm than would be most efficient, which results in greater fuel consumption, greater engine wear, and more noise than would result from operation at the engine's most efficient rpm. The operation of such systems in the prior art has been made more efficient by incorporating a two-speed transmission, but at higher cost, weight and axial length.
Many marine jet propulsion systems of the prior art feature a direct connection between the pump and the engine to eliminate the cost and axial length of a transmission or clutch. In these designs of the prior art, the neutral position that could be provided by the transmission or clutch is approximated by partially reversing the flow from the jet. The operator cannot easily maintain the balance of this partial reversing, especially given the sudden surge when starting the engine, so that the watercraft moves unpredictably. A true neutral control position would be desirable to enhance the operator's control of the watercraft.
Trash management is another shortcoming of the marine propulsion systems of the prior art. Many types of floating debris can become lodged on the grate that covers the inlet of the system, which restricts the flow of water into the pump and reduces propulsion efficiency. There are three types of such debris: solid objects, like rocks; fibrous material, like rope, fishing line, grass, reeds, and the stems of aquatic plants; and sheet material that can blind large sections of the grate, like large kelp leaves and plastic bags. The fibrous material is also well known to lodge on the leading edges of pump and stator vanes, reducing pump efficiency. The rope is particularly difficult to disentangle, when it becomes wrapped around the impeller and the drive shaft. Some jet boats carry hand rakes with right angle bends in the handle to remove debris from the inlet grate, and some integrate moveable grate sections to remove such debris, but these methods are awkward and only partially effective. Some commercial water jet propulsion systems are equipped with a reversing transmission, which is used to back flush both the pump vanes and the grate. As a last resort, commercial systems and river boats are commonly equipped with a clean-out hatch, which can be removed to allow the operator to remove debris from the pump inlet by hand. It would be desirable to reduce or eliminate the need for the trash handling mechanisms and methods by providing trash handling and back flushing methods integral to the design of the marine jet propulsion system.
In the marine jet propulsion systems of the prior art, reverse thrust is achieved by redirecting the water jet back under the boat along hydraulic reaction surfaces. Such reaction surfaces are commonly carried on a structure known as a “bucket”, which is mechanically moved into the jet stream by the operator to get reverse thrust. Buckets for large jets take up considerable space and add weight and cost to the system. It would be desirable to eliminate the need for the bucket by incorporating a method of producing reverse thrust in the pump design.